* When targeting SPIR-V 1.5, using gl_ViewportIndex will emit OpCapability ShaderViewportIndex and using gl_Layer will emit OpCapability CapabilityShaderLayer. OpCapability ShaderViewportIndexLayerEXT will only get emitted if the target < SPIR-V 1.5

* When using one of the viewport/layer arrays extensions, fallback to OpCapability ShaderViewportIndexLayerEXT, even when targeting SPIR-V 1.5

* Revert "When using one of the viewport/layer arrays extensions, fallback to OpCapability ShaderViewportIndexLayerEXT, even when targeting SPIR-V 1.5"

This reverts commit dccca82f4076ea6e2bc01dd6d1e5d290c59fab20.

* Using gl_Layer and gl_ViewportIndex outside of the geometry shader stage still requires one of the viewport extensions even when targeting SPIR-V 1.5
(Fixes a problem introduced by 670536b663f396815645b2f907f0ee92117b44f0)